nir: merge some basic consecutive ifs

After trying multiple times to merge if-statements with phis between them I've come to the conclusion that it cannot be done without regressions. The problem is for some shaders we end up with a whole bunch of phis for the merged ifs resulting in increased register pressure.

So this patch just merges ifs that have no phis between them. This seems to be consistent with what LLVM does so for radeonsi we only see a change (although its a large change) in a single shader.